count pairs given sum

Author: sachuverma

DSA Crack Sheet/README.md

@@ -18,6 +18,7 @@
 - [Next Permutation](https://leetcode.com/problems/next-permutation/ "view question") - [Cpp Solution](./solutions/Next%20Permutation.cpp)
 - [Count Inversions](https://practice.geeksforgeeks.org/problems/inversion-of-array-1587115620/1# "view question") - [Cpp Solution](./solutions/Count%20Inversions.cpp)
 - [Best Time to Buy and Sell Stock](https://leetcode.com/problems/best-time-to-buy-and-sell-stock/ "view question") - [Cpp Solution](./solutions/Best%20Time%20to%20Buy%20and%20Sell%20Stock.cpp)
+- [Count pairs with given sum](https://practice.geeksforgeeks.org/problems/count-pairs-with-given-sum5022/1# "view question") - [Cpp Solution](./solutions/Count%20pairs%20with%20given%20sum.cpp)
 - []( "view question") - [Cpp Solution](./solutions/.cpp)
 
 <!-- - []( "view question") - [Cpp Solution](./solutions/) -->

DSA Crack Sheet/solutions/Count pairs with given sum.cpp

@@ -0,0 +1,48 @@
+/*
+Count pairs with given sum
+==========================
+
+Given an array of N integers, and an integer K, find the number of pairs of elements in the array whose sum is equal to K.
+
+Example 1:
+Input:
+N = 4, K = 6
+arr[] = {1, 5, 7, 1}
+Output: 2
+Explanation: 
+arr[0] + arr[1] = 1 + 5 = 6 
+and arr[1] + arr[3] = 5 + 1 = 6.
+
+Example 2:
+Input:
+N = 4, X = 2
+arr[] = {1, 1, 1, 1}
+Output: 6
+Explanation: 
+Each 1 will produce sum 2 with any 1.
+Your Task:
+You don't need to read input or print anything. Your task is to complete the function getPairsCount() which takes arr[], n and k as input parameters and returns the number of pairs that have sum K.
+
+Expected Time Complexity: O(N)
+Expected Auxiliary Space: O(N)
+
+Constraints:
+1 <= N <= 105
+1 <= K <= 108
+1 <= Arr[i] <= 106
+*/
+
+int getPairsCount(int arr[], int n, int k)
+{
+  unordered_map<int, int> freq;
+  for (int i = 0; i < n; ++i)
+    freq[arr[i]]++;
+  int ans = 0;
+  for (int i = 0; i < n; ++i)
+  {
+    ans += freq[k - arr[i]];
+    if (k == 2 * arr[i])
+      ans--;
+  }
+  return ans / 2;
+}